Interpreting Seismic Signals Associated With Active Volcanic Systems

Abstract:

Volcanoes are the sources for a rich variety of seismic signals. Different types of events reveal different physical processes. A commonly observed sequence of events prior to eruptions, known as the generic volcanic earthquake swarm model (GVESM), provides a unifying framework. First to appear are deep long-period (DLP) events, which are thought to represent fluid injection at depth. From a background rate, high-frequency (HF) events (also called volcano-tectonic or VT events) increase in rate and often occur in swarms. The likely source for these events is shear failure on pre-existing faults. Increasing pressure manifests as a decrease in the b-value, whereas an increase in b-value may indicate higher thermal gradient or higher pore pressure. Thus the seismicity may indicate both changing processes as well as the state of the crust. Shoaling of HF earthquakes has been observed at a few volcanoes, but is not as common as one might expect. Instead, the appearance of hybrid and low-frequency (LF) events often indicates that magma has reached shallow depths. These events likely represent fluid involvement, either by a fracture adjacent to a fluid cavity (hybrids) or vibration of the fluid cavity itself (LF events). They are most common at shallow depths of 3 km or less, and swarms of such events typically have high b-values, suggesting high thermal gradients, high pore pressure, or highly fractured material. Shallow very-long period (VLP) events have been modeled as pulses of fluid flow in both dikes and sills. Moment tensor solutions can help distinguish the geometries. Volcanic tremor also generally occurs at shallow depths, and often has similar frequency content to LF events, suggesting a common source. Weak tremor is characteristic of hydrothermal activity, whereas stronger tremor suggests magmatic involvement. The reduced displacement of tremor is proportional to the VEI once the eruptions are under way. In some cases, deep high-frequency events occur after eruptions, suggesting that stresses have been modified in the country rock in the vicinity of the magma chamber.

The suite of different events comprising the GVESM can be used to infer that different processes are occurring, and the processes combined with different depths suggest that the underlying cause of the GVESM is the ascent of magma.